The Midtown high-rise fire that killed an aspiring playwright spread outside the original apartment because the tenant ran when he saw smoke and left his door wide open, authorities said Monday.

“When the occupant returned from shopping and he went to enter his apartment, he opened the door, it was full of smoke, he left, and the door remained in an open position,’’ FDNY Chief of Operations James Esposito said outside the luxury 42-story The Strand on West 43rd Street. “The door did not self-close’’ as it should have, Esposito went on.

“The fire received all the oxygen it needed to become a free-burning fire.’’

An extension cord with an overloaded power strip appeared to have started Sunday’s 11 a.m. blaze, igniting a Christmas tree in the 20th-floor apartment, authorities said.

“It was up against a window so it vented out, made it [spread],’’ a fire source told The Post.

The condo owner did not return phone calls, nor did the building’s management company, Firststate Residential.

Daniel McClung, 27, and his new husband, Michael Cohen, 32, became trapped in a smoke-filled 31st-floor stairwell. ­McClung died, and Cohen is hospitalized in “very serious condition,’’ Esposito said.

Two dogs also were killed. A neighbor said the couple had two pooches, a Chihuahua and a beagle, but it was unclear if they were the pets that died.

The couple lived on the 38th floor and were running down the stairs along with some other residents — the opposite of what tenants in a fireproof building should do, authorities said.

“They would have survived, absolutely, if they stayed in their apartment,’’ Esposito said.

Experts said that in buildings where there is concrete or other noncombustible material between units and only steel doors, residents should stay inside and jam something beneath the door to prevent smoke from getting in and open windows from the top.

Jim Bullock, president of the New York Fire Safety Institute, added to never lock your door if you evacuate because firefighters would have to break it down as they check for people.